The present invention generally relates to pressurized water nuclear reactors (PWRs). The invention more particularly relates to a tool for extracting, severing and storing irradiated bottom-mounted instrumentation (BMI) flux thimbles from the reactor vessel.
A pressurized water nuclear power station contains a closed loop of pressurized water which heat energy from the core of the PWR and transfers the energy to a second system for generating steam. The steam, in turn, drives a turbine generator which produces electrical power. The PWR consists of a reactor vessel (RV) containing the nuclear fuel which generates the heat energy, a steam generator in which the heat energy is sued to generate team, a circulating pump which circulates the coolant, and a pressurizer that maintains and controls system pressure.
FIG. 1 depicts a typical reactor vessel 74 of a PWR assembly. The reactor vessel 74 has a thermal shield, a core, support plates, control rods, etc. Coolant enters the vessel and flows down an annulus between the core barrel and the vessel wall; in the process in cools the thermal shield. The coolant turns, flows up through the fuel elements and out of the vessel to a steam generator. Neutron flux detectors extend through the bottom head into the core and are combined with thermocouples which measure coolant temperature inside the core; together they are known as "incore instrumentation." The core instrumentation provides data with which the power produced in different regions of the core in calculated.
The fuel rods typically arranged in grids. One assemblage or grid or fuel rods is called a "fuel element assembly", and is the smallest fuel unit handled in a power station. A core is built up by grouping fuel assemblies side-by-side; the higher the power level desired, the more fuel element assemblies are used. Core diameters typically run from 9 to 13 ft. (2.74-3.96 meters), depending on the power level. Core height is typically 10 to 14 ft. (3.05-4.27 meters) for large PWRs.
The neutron flux in the core is measured by the incore instrumentation while the reactor is operating. The flux thimbles 38, which contain the flux detectors, have a first end that extends into the core and a second end that enters a measurement room in the vicinity of the reactor core. The flux thimbles are slidably mounted in BMI guide tubes 36 of great length and are kept in a fixed position during reactor operation. The flux thimbles must be withdrawn from the fuel assemblies when the reactor core is being recharged. In a known embodiment, the flux thimbles pass through fitting on the convex of the RV 74. The guide tubes 36 connected to these fittings form a path, in the shape of a circular arc of large radius, joining the bottom of the RV to the measurement room.
A disadvantage associated with the arrangement just described is that the RV bottom fittings complicate manufacture of the RV and lead to difficulties in observing safety standards. Moreover, the structure of the reactor building must be designed to permit the passage of guide tubes of great length along a circular trajectory, which makes the design and construction of the reactor building more difficult and costly. Furthermore, gaining access to the fittings passing through the bottom of the RV is very difficult, which complicates monitoring these fittings to ensure that the reactor operates in complete safety.
In another known instrumentation arrangement, the flux thimbles pass through the RV closure head. This arrangement avoids the disadvantages described above; however, in an arrangement of this kind, a part of the instrumentation called "upper internal equipment" is carried directly by the closure head, which complicates dismantling the closure head and handling and storing the instrumentation during reactor stoppages. The upper internal equipment is withdrawn and arranged on a storage stand in the reactor pool during recharging and maintenance of the reactor. The arrangements in which the instrumentation guide tubes pass through the closure head make it impossible to simply and quickly handle the upper internal equipment. For further background on PWRs, see "Nuclear Power Plant Systems and Equipment," by K. C. Lish, ISBN 0-8311-1078-3.
The advantages obtained in the design and construction of the reactor when the core instrumentation passes through the RV closure head are thus accompanied by considerable disadvantages in the use and maintenance of the reactor; for this reason most PWRs now in service employ BMI instrumentation tubes. The flux thimbles in a MBI reactor are typically removed by first removing the nuclear fuel and then pulling the thimbles up through the RV and cutting them into pieces under water, which makes it impractical to replace individual thimbles as needed. The object of the present invention is to provide a self-contained tool for extracting and cutting up the thimbles without removing the fuel from the RV.